With a recent increase in a capacity of an information processing, various information recording techniques have been developed. In particular, a surface recording density of a hard disk drive (HDD) using a magnetic recording technique has been continuously increased at an annual rate of approximately 100%. Recently, there has been demanded an information recording capacity exceeding 60 Gbytes per 2.5-inch diameter magnetic disk to be used for the HDD. In order to meet the requirements, it has been demanded to implement an information recording density exceeding 100 Gbits per square inch.
In order to achieve a high recording density in the magnetic disk to be used for the HDD, it has been required to microfabricate a magnetic crystal grain constituting a magnetic recording layer for recording an information signal and to reduce a thickness of the layer.
In case of a magnetic disk using an in-plane magnetic recording system (which is also referred to as a longitudinal magnetic recording system or a horizontal magnetic recording system) which has been conventionally commercialized, however, the microfabrication of the magnetic crystal grain has been developed. As a result, a thermal stability of a recording signal is deteriorated by a superparamagnetic phenomenon so that a thermal fluctuating phenomenon is generated, that is, the recording signal is eliminated, causing an inhibition of an increase in a recording density of the magnetic disk.
In order to eliminate the factors of the inhibition, a magnetic disk for a vertical magnetic recording system has been proposed in recent years.
In case of the vertical magnetic recording system, an easy axis of a magnetic recording layer is regulated to be oriented in a perpendicular direction to a substrate surface, which is different from the in-plane magnetic recording system.
Since the vertical magnetic recording system can suppress the thermal fluctuating phenomenon more greatly than the in-plane recording system, it is suitable for an increase in a recording density.
In the vertical magnetic recording disk, a so-called two-layer type vertical magnetic recording disk including, on a substrate, a soft magnetic ground layer formed of a soft magnetic material and a vertical magnetic recording layer formed of a hard magnetic material is preferable.
In the two-layer type vertical magnetic recording disk, it is possible to form a suitable magnetic circuit among the magnetic head, the vertical magnetic recording layer and the soft magnetic ground layer in magnetic recording, and the soft magnetic ground layer serves to help the magnetic recording onto the vertical magnetic recording layer.
As the two-layer type vertical magnetic recording disk, for example, there has been known a vertical magnetic recording medium described in JP-A-2002-74648 (Patent Document 1).
Moreover, JP-A-2002-92865 (Patent Document 2) has disclosed a technique related to a vertical magnetic recording medium in which a ground layer, a Co based vertical magnetic recording layer and a protecting layer are formed on a substrate in this order. Furthermore, U.S. Pat. No. 6,468,670 Specification (Patent Document 3) has disclosed a vertical magnetic recording medium having a structure in which an artificial lattice film continuous layer (a switched connection layer) that is subjected to a switching connection to a particulate recording layer is bonded.    Patent Document 1: JP-A-2002-74648    Patent Document 2: JP-A-2002-92865    Patent document 3: U.S. Pat. No. 6,468,670